Motion due to the respiratory cycle (i.e., inhalation and exhalation of the lungs) can cause severe distortion in the geometry of target tissue of interest during a free-breathing static computed tomography (CT) scan. Free-breathing generally means the patient breathes during scanning. That is, the patient is not required to hold their breath. Static refers to the patient support being held at a same static position for the scan. The motion induced distortions can randomly shorten or lengthen the target tissue of interest. The distortions can also dislocate the center of the target tissue of interest. FIG. 1 show geometric distortion of an object of interest in a static CT image.
Because of these distortions, a free-breathing static CT scan is not well suited for dose planning for radiation therapy, especially for lung cancer tumors. 4D CT is an approach which mitigates this shortcoming. In 4D CT, the patient is over sampled along his/her long axis at every subject support position of interest. Each CT slice is then correlated with a breathing phase of the respiratory cycle. The CT slices with similar breathing phase but acquired at different couch positions are binned together, sorted based on the couch position, and concatenated into a 3D image. As such, 4D CT volume is a series of static CT images acquired at different breathing phases.
In order to measure the breathing phase, a surrogate respiratory signal is used. One surrogate is determined via spirometry. In spirometry, the flow of air in and out of the lung is measured by breathing through a device that has a turbine-shaped fan enclosed in a tube. The rate of rotation of the fan determines the air flow rate and is measured as a respiratory signal. Another surrogate is to track reflective markers placed on the patient's chest or abdomen. The reflective markers move as the patient breath and their motion can be used as a respiratory signal. Another surrogate is an air-bellow belt that captures the change in abdomen size during breathing.
Unfortunately, the above-noted approaches do not correlate well with the actual breathing phases of the respiratory cycle of a patient. As a consequence, 4D CT volumes often contain motion artifacts. FIG. 2 shows an example of motion artifact over four different phases of a 4D CT where the target is depicted at different locations in each image. FIG. 3 shows a 4D CT image of the lungs, chest, and shoulder of a patient with example motion artifact 302. In view of at least the above, there is an unresolved need for another approach for determining a motion signal that correlates well with the actual breathing phases of the respiratory cycle of a patient.